They got the fine because a team member didn’t follow the orders of a marshal, not because they fiddled with the marker.

After the formation lap had started on Sunday, a Red Bull team member re-entered this gate well area as the pit marshals started to close the gate and did not react when they tried to stop him.

[…]

There is nothing wrong with McLaren using tape or a visual marker on the wall at the side of the track as it is not governed by any regulation around the start procedure or the use of the grid box.

That also means it is not protected by the rules so another team interfering with it once it is there is legal, although whether it is in the spirit of competition would be open to interpretation.